Communication systems, such as two-way cable television systems, typically process signals in both the forward, or downstream, direction and the reverse, or upstream, direction. These signals may travel long distances and, as a result, distribution amplifiers are typically employed to amplify the signal levels of the forward and reverse signals. The lines that carry the signals have losses that vary with temperature. In addition to the lines, the performance of amplifiers within a communication system varies with the temperature. A cable distribution system is especially sensitive to temperature changes since the amplifiers and the coaxial cable are at ambient temperature, which can fluctuate greatly during a day or seasonally. Some communication systems consequently have to be operable over a range of temperatures from −40.degree. C. to +60.degree. C. As a result, the signal output varies with temperature and, as a result, fluctuations in attenuation across the system can introduce distortion into the signal.
To minimize the fluctuations based upon temperature, conventional amplifiers have used thermal circuits to detect the temperature within the amplifier. As the temperature within the amplifier rises, a variable attenuator, such as a Bode circuit, is adjusted to boost the gain of the amplifier. So configured, the circuit compensates for the decrease in gain that is a normal consequence of higher temperatures. Thermal circuits, however, are relatively imprecise and may not be able to compensate for other causes of gain variation. An example of such an arrangement can be found in U.S. Pat. No. 4,812,779, to Wagner.
A similar configuration of a representative block diagram of a thermally controlled amplifier 6 is depicted in FIG. 1. Because to include them will not enrich the discussion, the details of the Level Sensing Circuit 6 are not shown. While relatively effective, the thermal circuit was not based solely upon detected levels in the signal but rather relies, as well, on thermal condition of the amplifier itself and the known response to it. Important building blocks are present as well. First, there is a signal conditioner and comparator 11 that compares the strength of a gain request voltage that the Level Sensing Circuit 6 produces as indicia of signal strength. The comparator 11 compares the from a known signal source 7 (in this case a manual designate reference signal), and as a result formulates a “gain up” or “gain down” signal it passes to the Equalizer Driver 13, which in turn directs the Equalizer 17 to amplify or attenuate the input to produce an output signal that is relatively stable even in light thermal fluctuations.
Additional accuracy in signal processing can be achieved through automatic amplification and attenuation that monitors a signal level of a distinct pilot signal and then attenuates or amplifies the received RF signal in accord with the amplitude of the received pilot signal so the received RF signal maintains a constant and optimum level so the signal suffers minimal distortion as it is processed by the receiver. Automatic gain control (AGC) is an adaptive system found in many electronic devices including amplifiers.
While either a digital or an analog pilot signal will work for purpose of most AGC circuits, generally, customers such as providers of cable television do not wish to be tied to an analog pilot signal. Referring, for example, to the actual waveforms as shown in FIG. 2. NTSC 26, or analog television signals occupy a very narrow portion 27 of an allocated channel in the spectrum. On the other hand, a QAM channel 29 occupies a wider portion 28 of the allocated channel.
Maintaining an analog carrier for the purpose of enabling AGC is inefficient, as the same 6 MHz occupied spectrum could carry several digital services. To that end, it is extremely desirable to develop an AGC circuit based upon a digital signal known as a quadrature amplitude modulated (QAM) pilot signal.
Because of the need for AGC circuitry in system amplifiers, such as those used in the distribution of cable television through coaxial networks, the electronics industry has adopted a standard form factor to a package known as a Compact AGC module for insertion into amplifiers and thereby to enable AGC functionality in such amplifiers. In the present form factor, these AGC modules plug into sockets configured to receive the form factor into physical and electronic incorporation into the amplifiers.
Conventionally, two sorts of AGC circuits are available in a compact AGC module. First, a single pilot level sensing is used on a single channel and is conditioned by a narrow-pass ring. The advantage of a single pilot level is that its gain control is the more accurate of the two types. Generally, they used a fixed channel and often NTSC channels. Single pilot level sensing is recommended for trunk or buried plants. The single pilot AGC circuit is recommended for medium to long cascades. A representative example of the Single Pilot Compact AGC Module 8 is depicted in FIG. 3. Shown here, in addition to the components shown in the thermal sensing circuit unit is a Narrow Band Filter 10. An ideal Narrow Band Filter 10 would have a completely flat narrow band (e.g. with no gain/attenuation throughout) and would completely attenuate all frequencies outside the designated narrow band. Additionally, the transition out of the narrow band would be instantaneous in frequency giving steep and immediate attenuation outside of the designated narrow band.
In practice, no Narrow Band Filter 10 is ideal. The filter 10 does not attenuate all frequencies outside the desired frequency range completely; in particular, there is a region just outside the intended Narrow Band where frequencies are attenuated, but not rejected. Nonetheless, filtering an incoming signal with a Narrow Band Filter yields a signal generally proportionate to the signal strength within the narrow band. Where the narrow band is selected to be a pilot signal, the gain request voltage provided to the Comparator 11 reflects the power received in the pilot band frequency. All other components function just as they do in the Thermally Controlled Circuit 6 shown in FIG. 1.
A second type is known as composite pilot level AGC. Using a whole composite carrier as a pilot rather than a single channel. Composite AGC employs a wideband filter primarily for economic purposes. A wideband filter is easier to make and cheaper to build. Using a composite gives a larger part of the spectrum and, thus, detected power is about 5 times, or about 7 dB, greater than a single pilot AGC. Loss of even one of the pilot channels means a concurrent loss of 1 dB of error in gain control due to that loss of signal strength. Composite pilot sensing allows the AGC function at lower cost than single pilot sensing types. The composite AGC also performs its signal attenuation and gain with lower accuracy than single pilot sensing types and to perform optimally, the composite AGC requires careful balancing of 5 pilot channels for proper operation. The composite AGC is useful for trunk or buried plant and can be used in a cascade configuration. The composite pilot AGC is portrayed in FIG. 4.
In comparing FIGS. 3 and 4, it is notable that both types of AGC exploit the same equalizer circuit (and loss) as single pilot comprising a Comparator 11, an Equalizer Driver 13 and an Equalizer 15. Because the signal strength in a received signal, once sensed, either by single or composite pilot means that a voltage resulting from a received signal is presented to a Comparator 11 against a manually designated level in accord with a reference voltage generated at the Manual Level Set 7. Where the sensed signal strength voltage exceeds the manual level, Equalizer Driver 13 drives the Equalizer 15 to attenuate the incoming signal. Similarly, where the sensed strength of the signal results in a smaller voltage at the comparator, the Equalizer Driver 13 instructs the Equalizer 15 to amplify the signal. The process of always comparing an output to a reference signal is known as a closed loop feedback control and in this case is based upon a received voltage representing the signal strength of the pilot signal at a receiver.
What is absent in the art is an automatic gain control circuit that can detect a narrow band that is designatable or frequency nimble in order to detect a signal in a received spectrum without the use of an expensive Narrow Band Filter 10.